The basic steps in the manufacture of sheet glass, e.g., sheet glass for use as substrates for displays, such as LCD displays, include: (1) melting raw materials, (2) fining (refining) the melt to remove gaseous inclusions, (3) stirring the fined molten glass to achieve chemical and thermal homogeneity, (4) thermally conditioning the homogenized glass to reduce its temperature and thus increase its viscosity, (5) forming the cooled molten glass into a glass ribbon, and (6) separating individual glass sheets from the ribbon. In the case of a downdraw fusion process, the glass ribbon is formed using a forming structure known as an “isopipe,” while in a float process, a molten tin bath is used for this purpose.
High temperatures are needed to successfully fine molten glass since the rate of rise of gaseous bubbles through molten glass varies inversely with the viscosity of the glass, i.e., the lower the viscosity, the faster the rate of rise, and the viscosity varies inversely with temperature, i.e., the higher the temperature, the lower the viscosity. Because molten glass is in a finer for only a limited amount of time, achieving a rapid rise of bubbles through the melt is of great importance. Hence, the finer is normally operated at as high a temperature as possible. However, to form molten glass into a ribbon requires viscosities much higher than those used during fining. Hence, the need to thermally condition (cool) the molten glass between fining and forming.
Historically, thermal conditioning has been performed by passing the molten glass through a conduit having a circular cross-section. The conduit has been surrounded by ceramic material and held in a metal frame, and the rate of heat loss from the molten glass has been controlled through the use of direct or indirect heating so as to avoid introducing substantial thermal and flow inhomogeneities into the glass as a result of the cooling process. Because of the high temperature of the molten glass and the need to avoid contamination of the molten glass, the wall of the conduit has been made of a precious metal.
The circular cross-section used in the past has provided a conduit that is intrinsically mechanically stable. Such stability is important because precious metals are expensive and thus to reduce cost, the wall of a conditioning conduit needs to be as thin as possible. Although good for mechanical stability, in accordance with the present disclosure, it has been found that a circular cross-section is not the best in terms of heat transfer. Specifically, it has been found that for various applications, because of their heat transfer properties, conditioning conduits having circular cross-sections need to be longer than the available space. Although it is possible to extend the space used for thermal conditioning, such extensions increase the overall size and thus the cost of the glass manufacturing facility. In addition, increasing the length of a circular conditioning conduit increases the amount of precious metal needed to construct the conduit, thus diminishing the economic benefits associated with the thin walls of such conduits.
The present disclosure addresses this issue with circular conditioning conduits and provides conduits, specifically, oblong conduits, which achieve high rates of heat loss without compromising the transverse thermal flow homogeneity (transverse thermal uniformity) of the molten glass passing through the conduit. In addition, the flow gradients and head losses of molten glass passing through the conduits of the present disclosure are small, which are added benefits of the oblong conduits.